These messages appear when I'm rendering (and overwriting) a C4d-animation that was placed in a Composition. AE should be a bit less radical, and just stop trying to reveal the footage (replace it by a placeholder) and let the user decide when to reload it. (normally when external rendering is done).
At the moment the only remedy is restarting, or waiting for the render to finish.
Btw: the terminology used gives a peak at the heart of AE's code. It calls this situation 'input contract violation'. The user is punished with a frozen app...
